The present invention relates to light emitting devices and, more particularly, to light emitting devices that have high color rendering index (“CRT”) values.
Light emitting diodes (“LEDs”) are well known solid state lighting devices that are capable of generating light. LEDs generally include a plurality of semiconductor layers that may be epitaxially grown on a semiconductor or non-semiconductor substrate such as, for example, sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrates. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will “collide” with a hole and recombine. Each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the electrons and holes recombine).
LEDs typically have a narrow wavelength distribution that is tightly centered about a “peak” wavelength (i.e., the single wavelength where the radiometric emission spectrum of the LED reaches its maximum as detected by a photo-detector). For example, the spectral power distributions of a typical LED may have a full width of, for example, about 10-30 nm, where the width is measured at half the maximum illumination (referred to as the full width half maximum or “FWHM” width). Accordingly, LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Thus, the dominant wavelength differs from the peak wavelength in that the dominant wavelength takes into account the sensitivity of the human eye to different wavelengths of light.
As most LEDs are almost monochromatic light sources that appear to emit light having a single color, LED lamps that include multiple LEDs that emit light of different colors have been used in order to provide solid state light emitting devices that generate white light. In these devices, the different colors of light emitted by the individual LED chips combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue light emitting LEDs, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the source red, green and blue LEDs.
White light may also be produced by surrounding a single-color LED with a luminescent material that converts some of the light emitted by the LED to light of other colors. The combination of the light emitted by the single-color LED that passes through the wavelength conversion material along with the light of different colors that is emitted by the wavelength conversion material may produce a white or near-white light. For example, a single blue-emitting LED chip (e.g., made of indium gallium nitride and/or gallium nitride) may be used in combination with a yellow phosphor, polymer or dye such as for example, cerium-doped yttrium aluminum garnet (which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce), that “down-converts” the wavelength of some of the blue light emitted by the LED, changing its color to yellow. Blue LEDs made from indium gallium nitride exhibit high efficiency (e.g., external quantum efficiency as high as 60%). In a blue LED/yellow phosphor lamp, the blue LED chip produces an emission with a dominant wavelength of about 450-460 nanometers, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nanometers in response to the blue emission. Some of the blue light passes through the phosphor (and/or between the phosphor particles) without being down-converted, while a substantial portion of the light is absorbed by the phosphor, which becomes excited and emits yellow light (i.e., the blue light is down-converted to yellow light). The combination of blue light and yellow light may appear white to an observer. Such light is typically perceived as being cool white in color. In another approach, light from a violet or ultraviolet emitting LED may be converted to white light by surrounding the LED with multicolor phosphors or dyes. In either case, red-emitting phosphor particles (e.g., a CaAlSiN3 (“CASN”) based phosphor) may also be added to improve the color rendering properties of the light, i.e., to make the light appear more “warm,” particularly when the single color LED emits blue or ultraviolet light.
As noted above, phosphors are one known class of luminescent materials. A phosphor may refer to any material that absorbs light at one wavelength and re-emits light at a different wavelength in the visible spectrum, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Accordingly, the term “phosphor” may be used herein to refer to materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors may absorb light having first wavelengths and re-emit light having second wavelengths that are different from the first wavelengths. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths.
LEDs are used in a host of applications including, for example, backlighting for liquid crystal displays, indicator lights, automotive headlights, flashlights, specialty lighting applications and even as replacements for conventional incandescent and/or fluorescent lighting in general lighting and illumination applications. In many of these applications, it may be desirable to provide a lighting source that generates light having specific properties.